The present invention relates to an installation for the wet grinding of water-soluble polymers to obtain their dispersions and then subsequently, their rapid dissolution in water. It also relates to a method implementing the device.
According to the invention, the polymer is put in suspension in the water by passing through a grinder comprising a rotor with knives rotating in a stator with very close blades; the plugging of this stator by the polymer gel formed being prevented by secondary water jets which disperse and dilute the polymer.
Among the water-soluble polymers belonging to the prior art, partially hydrolysed acrylamide polymers and their copolymers are particularly known, and also xanthan gums, cellulose derivatives and guar gums. These polymers develop a viscosity thanks to their molecular weight and/or the inter-chain ionic repulsions. The mechanism governing the viscosity is linked to a rise in hydrodynamic volume or to inter-chain repulsions.
Although acrylamide (co)polymers are usually available commercially in the form of powders, they are generally used in dilute aqueous solutions in industrial applications. This necessitates a step of dissolution of the polymer in the water in precise physical and chemical conditions.
However, even if these polymers are hydrophilic, their dissolution is difficult. Their dissolution varies, in particular, according to their composition and their molecular weight.
Thus, to be used in solution, powder polymers are first dispersed in water using wetting equipment. The main equipment used is of various types:                eductor with dry or wet feed hopper,        water/air disperser in which the polymer is transported by an air actuated system in a chamber where it is wet by spray nozzles,        various high speed stirrers.        
The dispersion in water thereby obtained is then dissolved continuously or in batches by stirring.
The main drawback of these dispersion systems is that, the higher the molecular weight of the polymer, the higher the viscosity of the resulting solution. This has the consequence of limiting the polymer concentration in the water, generally from 1 to 5 grams/liter, and therefore requiring extremely large dissolution tanks for large scale industrial applications.
For example, for high molecular weight acrylamide (co)polymers (about 15 million), in powder form with an average particle size of 0 to 1 mm, at the temperature of 20° C., the dissolution time required to obtain a solution of 5 g/l is about:                4 hours for a nonionic polymer,        1 hour for an anionic polymer,        45 minutes for a cationic polymer.        
To solve these problems of concentration, dispersion/dissolution and equipment cost/size, various methods have been developed. The main methods are listed below. They are based on two guidelines:—modification of the commercial form of the polymer (cf 1-5) and—improvement of the dissolution equipment (cf 6).
1/Reduction of the Size of Particles with Standard Grain Size Distribution by Dry Grinding.
Acrylamide (co)polymers in solid (powder) form are mainly produced by gel polymerisation followed by steps of chopping, drying then grinding. It is well known to a person skilled in the art that a significant action on the grain size distribution of the powder (decrease) has the result of facilitating its hydration and hence its dissolution.
However, this solution has many limiting factors, that is:                high grinding cost: because the softening point is low (close to 50° C.) requiring the use of large grinders with large quantities of cooling air or the use of cryogenic systems,        a very dusty end product: since the product is used directly by the operators, it is difficult to control the rate of particles in the atmosphere without sophisticated equipment,        an increase in ‘fisheyes’: during their use, the fine polymeric particles tend to cake together on wetting by forming numerous ‘fisheyes’. These are gelatinous particles ranging in size from a few millimeters to a few centimeters, and which only dissolve after several hours to several days. These aggregates tend to plug the lines, the metering pumps and the filters.2/Inverse Emulsion (Water-in-Oil) Polymerisation        
In that well known process to a person skilled in the art, the monomers are emulsified by a hydrocarbon and polymerised in the presence of stabilising surfactants. To dissolve the polymer thus obtained, it is necessary to add an inverted surfactant (high HLB) either directly to the emulsion or during its dissolution. The end product therefore has a high concentration of detrimental surfactants and an organic phase, resulting in a significant additional cost of raw materials (30 to 60%), transport (30 to 40%) and storage. This means that the emulsions are widely used for low- or medium-consumption applications because of their ease of use, but are too expensive for large scale projects.
3/Aqueous Dispersion Polymerisation (Also Called ‘Water-in-Water Emulsion’)
This technique consists in polymerising a monomer or a mixture of monomers in water containing a salt and/or other chemical agents in solution or in dispersion. The hydrophilic polymer formed during the polymerisation precipitates when it reaches a sufficiently high molecular weight. At the end of the polymerisation, a liquid dispersion of polymer particles in suspension in the aqueous mixture is recovered. The advantages of this technology are obvious. As to their manufacturing cost, this remains low, that is, similar to that of powder polymers, because the dispersion obtained comprises almost exclusively polymer, water and salts. Moreover, it has the same decisive advantage as the oil-in-water type emulsion, that is, very rapid solubilisation of the polymer in water.
These products nevertheless face several obstacles to their development:                a low concentration (15 to 20%) and hence extra cost for transport and storage,        limited polymer molecular weight,        reduced shelf life.4/Suspension Polymerisation        
This polymerisation method consists in forming droplets of an aqueous solution of the monomer or monomers in suspension in an inert liquid which, after polymerisation by addition of a catalyst, yield polymers in the form of beads. At the end of polymerisation, the water is then removed during an azeotropic distillation step. The polymer beads are then filtered and dried. The azeotropic distillation step is generally considered as critical. With this method, the particle size (100 to 400 microns) can be reduced nearly uniformly without forming large quantities of fines.
Here also, this solution has many limiting factors, that is:                the polymeric particles formed by this method also have a strong tendency to form fisheyes above a certain concentration,        moreover, the major limit of this method is the inability to produce very high molecular weights through its use. The molecular weights of the resulting polymers are limited to 10-12 million, which is insufficient in many industries.5/Placing the Powder in Suspension in Surfactants        
The finely ground polymer powder can be placed in suspension either in a hydrocarbon containing large quantities of surfactants, or directly in pure surfactants. These suspensions are rapidly dissolved but are unstable and have the same economic drawbacks as reverse emulsions.
6/Wet Grinding of Polymer Powder Having a Standard Grain Size Distribution
The standard grain size distribution polymer is placed in suspension in the water and then ground. To do this, documents U.S. Pat. No. 4,845,192, U.S. Pat. No. 4,877,588 and U.S. Pat. No. 4,529,794 describe a device comprising a closed cage equipped with fixed and moving knives (mounted on a rotor) and positioned at a spacing of 50 to 500 microns, with a clearance of 50 to 500 microns, which cut the product into very small particles, typically smaller than 200 microns. This apparatus is manufactured by URSCHEL under the trade name Comitrol. The Comitrol 1500 has a cutting diameter of 200 mm.
According to this method, it is the cutting dimension that determines the final dissolution time. Hence this method, which provides a significant improvement in dissolution time of water-soluble polymers after dispersion in the device appears to be advantageous. However, it has many major drawbacks:                the spacing of the knives and their angle is critical for obtaining a satisfactory cutting,        the speed required for satisfactory operation is very high: 10 000 to 13 000 rpm (e.g.: Comitrol 1500 apparatus equipped with an 8-inch rotor). At lower speed of rotation of the rotor, the system is blocked by plugging of the interval between the fixed knives: no dissolution is then possible,        the wear of the fixed and mobile knives is extremely rapid. On average, after continuous in-line use, it has been found that the knives had to be replaced every 10 to 90 days. This has the consequence of requiring the doubling of the number of grinding apparatus necessary and requiring difficult and lengthy maintenance due to the replacement of about 200 knives in very precise conditions and often beyond the scope of local maintenance personnel. This aging also occurs when using high strength materials,        furthermore, at these speeds, rapid aging and overheating of the bearings are observed, making this apparatus incompatible with ATEX standards (relative to workplace equipment safety). Ceramic bearings could diminish the problem without solving it,        finally, the installed motor capacities are extremely high, for example 30 kW for a Comitrol 1500.        
Due to these drawbacks which appear prohibitive, in 20 years, the use of this type of apparatus for dispersing water-soluble polymers has not spread.
The invention overcomes all the above mentioned drawbacks.